Production of aromatic hydrocarbons

Abstract

A process for the dimerization and cyclization of isobutene to form, as preferred product, paraxylene in which the whole of the product of isobutene dimerization (including unreacted feedstock) is contacted with a catalyst for the cyclization of the isobutene dimer. The process is operable in a single dimerization/cyclization stage or in separate dimerization and cyclization stages with no intermediate separation of products. Optionally, the isobutene feedstock is provided by dehydrogenation of isobutane, unreacted isobutane in the resulting feedstock being carried through the dimerization/cyclization and eventually recycled.

We claim: 
     
       1. A process for the production of aromatic hydrocarbons which comprises contacting a feed comprising isobutene at elevated temperature and in the presence of added oxygen with a catalytic amount of a catalyst for the oxidative dimerisation of olefins, to form a product comprising at least one dimer of isobutene and contacting the whole of said product (including any unreacted hydrocarbon feedstock) with a non-acid catalyst for the cyclisation of the isobutene dimer whereby to form a final product comprising aromatic hydrocarbons. 
     
     
       2. A process as claimed in claim 1 in which the process is operated in two sequential stages comprising a first stage for the oxidative dimerisation of isobutene and a second stage for the catalytic aromatisation of the dimer product from the first, dimerisation, stage. 
     
     
       3. A process as claimed in claim 2 in which the catalyst for the oxidative dimerisation stage is a metal oxide selected from one or more oxides of metals of Group VIII of the Periodic Table. 
     
     
       4. A process as claimed in claim 3 in which the catalyst comprises bismuth oxide which is present in a partially reduced state, the average degree of reduction throughout the whole bed being in the range 1 to 70%. 
     
     
       5. A process as claimed in claim 2 in which the degree of oxygen conversion, measured at the downstream end of the dimerisation catalyst bed is in the range 90 to 99%. 
     
     
       6. A process as claimed in claim 2 in which, in the dimerisation stage, the reaction temperature is in the range 400° to 600° C., the olefin partial pressure is in the range 0.20 to 0.99 atmospheres, and the oxygen partial pressure is in the range 0.01 to 0.30 atmospheres. 
     
     
       7. A process as claimed in claim 2 in which the aromatisation stage is carried out in the presence of a non acid catalyst selected from chromia-alumina, magnesium oxide, gallium, zinc, germanium, indium, tin and compounds of these metals, at a reaction temperature in the range 300° to 600° C. and in the absence of, or with a low concentration, of oxygen. 
     
     
       8. A process as claimed in claim 1 in which the process is operated as a single stage process using either a mixed or a dual-function dimerisation/cyclisation catalyst. 
     
     
       9. A process as claimed in claim 8 in which the catalyst is selected from bismuth phosphate, bismuth-tin-oxygen, bismuth-gallium-oxygen, bismuth-germanium-oxygen, bismuth-zinc-oxygen compositions and a mixture of chromia-alumina and bismuth oxide. 
     
     
       10. A process as claimed in claim 8 in which the catalyst comprises bismuth oxide in a partially reduced state, the average degree of reduction over the whole catalyst bed being in the range 1 to 70%. 
     
     
       11. A process as claimed in claim 8 in which the degree of oxygen conversion is in the range 90 to 99%, the partial pressure of oxygen is in the range of 0.01 to 0.30 atmospheres, and the partial pressure of olefin is in the range 0.20 to 0.99 atmospheres. 
     
     
       12. A process as claimed in claim 1 in which the isobutene feedstock is provided by a feed of isobutane which is subjected to dehydrogenation to convert it, at least in part to isobutene. 
     
     
       13. A process as claimed in claim 12 in which unreacted isobutane is separated with unreacted isobutene from the products of the dimerisation/aromatisation stage (one-stage process and aromatisation stage (two-stage process) and recycled to one or partially to both of (i) the isobutane dehydrogenation stage or (ii) the dimerisation/aromatisation stage or dimerisation stage. 
     
     
       14. A process as claimed in claim 1 in which unreacted isobutene is recycled to the dimerisation/aromatisation stage (one-stage process) or to the dimerisation stage (two-stage process).